Let me start by saying I, like most other snake hobbyists do not enjoy killing rats but it is a necessary evil. Some people use c02 or dry ice and what that does is essentially eliminate the oxygen in the enclosure. I use Ziploc bags, I put one small or just weaned rat in the bag and flatten the battle remove most to the air. The rat dies in 2-3 minutes and only moves erratically for 5-10 seconds so I believe the suffering is minimal. He can still breathe but the oxygen is quickly used up and they don't cry as they do when feeding live. Then they are already individually bagged I then weigh them and use a marker to write their weight on the bag. I have not used this on a full grown rat but I imagine you would need a pretty strong bag.